Conventionally, it is known that capacitor microphones can be manufactured in accordance with manufacturing processes used for semiconductor devices. Capacitor microphones are designed such that electrodes are attached to plates and diaphragms vibrating due to sound waves, wherein the plates and diaphragms are supported and distanced from each other by way of insulating spacers. Capacitor microphones convert capacitance variations due to displacements of diaphragms, included in capacitors constituted by plates and diaphragms, into electric signals. Sensitivities of capacitor microphones can be improved by increasing ratios of displacements of diaphragms in comparison with distances between electrodes, thus reducing leak currents of spacers and parasitic capacitances.                The document entitled MSS-01-34 published by the Institute of Electrical Engineers in Japan teaches a capacitor microphone in which both of a plate and a diaphragm vibrating due to sound waves are formed using conductive thin films. Due to the uniform rigidity of the diaphragm, even when the diaphragm propagates sound waves, only the center portion of the diaphragm vibrates with a maximum displacement, and the displacement due to the vibration of the diaphragm becomes small in a direction from the center portion to the outer periphery fixed to the spacer. That is, the other portions other than the center portion of the diaphragm having the uniform rigidity may reduce the sensitivity of the capacitor microphone. It may be possible to increase the sensitivity of the capacitor microphone by increasing the ratio of the maximum displacement of the diaphragm in comparison with the distance between the plate and the diaphragm. In this case, a bias occurs when the diaphragm approaches the plate so as to cause electrostatic absorption, by which the plate absorbs the diaphragm; in other words, there is a problem regarding the occurrence of a pull-in event.        
Japanese Patent Application Publication No. 2004-506394 (corresponding to WO2002/015636) teaches an example of a capacitor microphone (serving as an acoustic transducer) using a semiconductor substrate such as a silicon substrate. Herein, the outer periphery of a fixed electrode having a plate-like shape is fixed to an insulating layer formed on the semiconductor substrate so that the fixed electrode is supported by and bridged over the insulating layer, wherein a diaphragm electrode is supported in parallel with the fixed electrode with a relative distance therebetween, so that variations of the relative distance that occur when the diaphragm electrode vibrates due to sound waves are detected as variations of electrostatic capacitance.
In the aforementioned capacitor microphone, it is preferable that the fixed electrode be held in a fixed state with the insulating layer, and the diaphragm electrode be easily vibrated due to sound waves. Specifically, supports are extended inwardly from the insulating layer and are used to hang the diaphragm electrode at inner ends thereof so as to separate the diaphragm electrode from the insulating layer, thus realizing free deformation with respect to the diaphragm electrode.
In the manufacturing process of the capacitor microphone, tensile stress may remain in the diaphragm electrode, which is formed using a conductive film at a high temperature. Due to the tensile stress, the diaphragm electrode may be slightly bent or deformed, thus reducing the air gap between the diaphragm electrode and the fixed electrode. When these electrodes approach each other so as to be very close, these electrodes may come in contact with each other due to electrostatic attraction exerted therebetween, thus reducing a pull-in potential. In order to avoid the occurrence of a pull-in event, it is necessary to reduce the bias voltage applied to the capacitor microphone. Due to such a restriction, the manufacturer experiences difficulty in manufacturing high-sensitivity capacitor microphones.
Even though the diaphragm electrode is supported in a hanging state and is separated from the insulating layer, a terminal for applying voltage from an external device is extended from a part of the outer circumferential portion of the diaphragm electrode and is fixed to the insulating layer, whereby the diaphragm electrode is supported in an unbalanced manner such that it is hung downwardly by means of the support and it is also supported horizontally by way of the terminal fixed to the insulating layer. This makes the air gap (formed between the diaphragm electrode and the fixed electrode) become easily non-uniform, whereby the air gap may be reduced partially so as to cause a reduction of a pull-in potential. Such a problem causes another limitation in increasing the bias voltage applied to the capacitor microphone.
Furthermore, the non-uniform air gap and the fixation of the terminal interfere with vibration of the diaphragm electrode, which may cause asymmetrical deformation with respect to the center of the diaphragm. This produces dispersions of sensitivities and makes it difficult to predict the performance in designing.